1. Field of the Invention
This invention relates to turbine blade assemblies.
In particular, the invention relates to a fixed blade assembly for an axial flow turbine of the type wherein the assembly comprises a radially inner ring, a radially outer ring and a row of circumferentially aligned and spaced apart blades, each blade being secured at an inner end to the inner ring and at an outer end to the outer ring, and each blade having a leading edge, a trailing edge, a pressure side and a suction side.
2. Description of the Related Art
A fixed blade assembly of the type just defined followed by a set of blades attached to the turbine rotor, that is a set or row of moving blades, will constitute a turbine stage and the design of that stage will be concerned with efficiency of conversion of energy in a working fluid, for example steam, passing axially through that stage into mechanical energy in the rotor. The fixed blades are present in order to set up the flow for the moving blades and to optimise the energy extracted, but the fixed blades themselves introduce certain aerodynamic losses.
Known fixed blade assemblies of the type defined herein usually have straight blades, with each blade having a straight trailing edge on a radial line from the turbine axis. We have made and sold turbines with fixed blade assemblies of the type described in which a straight trailing edge of each blade is inclined to the flow, that is the straight trailing edge leans at an angle away from a radial line from the turbine axis through the inner end of that edge and in the direction that the pressure side faces, that is the straight trailing edge has a positive lean angle. We did this in the expectation that the increased fluid flow, from the fixed blade assembly to the inlets of the following row of moving blades, which would be induced at the inner (root) ends of the blades by the straight positive lean of the fixed blades would improve the stage efficiency. However, the hoped for efficiency improvement was not obtained.
An article by G. A. Filipov and Van Chzun-Tsi entitled "The effect of Flow Twisting on the Characteristics of Guide Rows" in Teploenergetika 1964,11,5,54-57, pages 69-73 considers the efficiency of fixed blade assemblies of the type defined herein in which the turbine flow section opens out from the inlet to the outlet of the fixed blade assembly. Consideration is given to straight radial blades, straight blades with positive lean and positively curved blades, that is to say blades which are curved circumferentially outward in the direction that the pressure side faces. The conclusion given is that the best hope for improved efficiency, subject to further experiment, should be the use of blades curved in this manner in combination with the provision of flow twisting at the inlet to the fixed blade assembly which it is suggested can be achieved by profiling the moving and fixed blades of the previous stage. We consider that a major deterrent to investigating this suggestion would be the extra cost in manufacturing a curved blade compared with the usual straight blade.
Published patent application GB 2199379A (General Electric Company) also considers the efficiency of fixed blade assemblies of the type defined herein and, as an alternative to the positively circumferentially curved blades above-mentioned in the Filipov article, proposes negatively circumferentially curved blades, that is to say blades curved circumferentially outward in the direction that the suction side faces. It is said that in this way the potential for low momentum airflow to enter the mainstream airflow is reduced, thereby reducing the magnitude of secondary losses. It is mentioned that these curved blades may additionally be leaned in the direction that the suction side faces, but there is no explanation or discussion of this suggested feature. Again, we consider that a major deterrent to investigating this General Electric Company disclosure would be the extra cost in manufacturing a curved blade compared with the usual straight blade.